The production of composite materials by the coating of layers of fluid substances onto solid substrates and solidifying the layers by drying or curing is well known. Composite layered materials formed by such coating processes are especially useful as information recording media.
An important requirement for layers of information recording media is uniformity. Non-uniformities are defects which can lead to improper information recording or retrieval. When information recording layers are formed by coating fluids, a common source of coating non-uniformity is the presence of oversized particulates in the coating fluid. These oversized particulates result in defects in the recording layer, and can be the result of contamination from outside the system, but are more commonly dried dispersion clumps formed within the coating fluid during coating. A common source of clumps is premature solidification of portions of the coating fluid in localized areas of the coating apparatus before the coating step.
Coating fluids used to produce magnetic recording layers, called magnetic recording fluids, typically include fine particles of magnetic materials, called magnetic pigments, dispersed in a liquid binder of polymeric materials, solvents, reactants, and catalysts. The liquid binder is formulated to solidify into a matrix which binds the pigment into a durable layer suitable for magnetic recording. Combinations of catalysts and other ingredients, called activators, initiate and sustain crosslinking or polymerization reactions during solidification. The properties of the resulting magnetic recording layer may be enhanced by additives such as lubricants, plasticizers, and antistatic agents.
The solidification of magnetic coating fluids into magnetic recording layers typically occurs first by evaporation of the solvent, then by chemical curing reactions such as crosslinking or polymerization. Solvent removal is initiated by coating the fluid as a thin film, since this greatly increases the surface area available for solvent evaporation. Curing is typically completed by the application of heat over time, which accelerates solvent evaporation and increases the rates of crosslinking and polymerization. Other forms of energy, such as ultraviolet light or electron beams, can promote crosslinking or polymerization.
While open reservoir coating apparatus are especially prone to localized premature drying of the coating fluid, these apparatus have many advantages which either outweigh the drying problem or provide great incentive to find solutions to it. Two typical types of reservoirs are the pan and the trough. In the pan type reservoir, shown in FIG. 1, a receiving roller, such as a gravure cylinder 10, is immersed in a coating fluid 12 in a reservoir 14. The gravure cylinder 10 rotates and carries a layer of coating fluid to a doctor blade 16, which contacts the gravure cylinder 10 and wipes off the excess fluid, leaving the remaining fluid on the gravure cylinder 10 to be carried to a substrate 18 on which it is coated. The substrate 18 is held close to the gravure cylinder 10 by a backup roller 20.
The amount of fluid 12 carried by the gravure cylinder 10 to the substrate 18 can be governed by providing small fluid-holding pits or grooves, called cells 32, in the outer surface of the gravure cylinder 10. By applying an abundance of coating fluid 12 to the gravure cylinder 10 and then wiping off the excess fluid with the doctor blade 16, the fullness of the cells 32 is controlled. Each cell 32 acts as a measuring cup so that the rate of coating fluid 12 application is closely controlled in both the downweb and crossweb directions.
Alternatively, smooth rollers, without cells, can be used and the coating thickness is controlled with a roller or a doctor blade. The roller or doctor blade is spaced a small distance from the surface of the roller to provide an accurately controlled gap for a layer of coating fluid to be carried by the surface of the coating roller. Transfer of the coating fluid to the substrate is similar to that found in celled gravure coating. Typical examples of controlling the application of coating fluid in this manner can be found in U.S. Pat. Nos. 4,864,930; 4,581,994; and 4,534,290. Alternatively, in offset roll coating, the coating fluid first is transferred to intermediate rollers and then is transferred to the substrate. Pan type reservoirs expose a large area of fluid to the air where solvent removal occurs, causing premature fluid drying. Pan systems also typically have stagnant areas near the pan walls where fluid residence time is long, resulting in further loss of solvent.
Referring to FIG. 2, trough type systems minimize the area of fluid that contacts the air and reduces fluid residence time. A suitable fluid level is maintained in the trough 22 by supplying excess coating fluid 12 to the reservoir 15 and providing an opening 28 in the reservoir wall at the height of the desired fluid level. Coating fluid fills the trough 22 to this level and overflow through a tube 30, returning to the reservoir 15.
In this configuration, the trough 22 is supplied with coating fluid 12 by a pump 24, which feeds the fluid from a reservoir 15 through a filter 26. The coating fluid 12 supplies a gravure cylinder 10, while the surface of the gravure cylinder 10 moves downwardly past the doctor blade 16. The coating fluid is then transferred to the substrate 18 by contact between the substrate 18 and the coating fluid 12 on the gravure cylinder 10, which is maintained by a backup roller 20. The excess fluid returns to the reservoir 15. Many trough systems use a top seal 34, shown in FIG. 2, which contacts the roller so the air volume above the fluid becomes saturated with solvent. These systems work only if the fluid remaining on the gravure roller does not contaminate the top seal. Many fluids dry on the seal causing flaws. If the system is run without the top seal or with the seal close to the supply roller, the air above the pool does not become saturated with solvent and the fluid in the reservoir dries.
An area in which this is particularly likely to occur is the region above the wetting line between the wall of the reservoir and the free surface of the coating fluid. Since the level of coating fluid in the reservoir is constantly changing, thin films of fluid can form on the reservoir walls as the fluid drains, due to dynamic wetting effects. This film can sometimes dry before the fluid level rises again, due to the increased rate of solvent evaporation brought about by the high surface area to volume ratios found in thin liquid films. When the fluid level rises and falls again, another layer of coating fluid is deposited over the first, adding to the thickness of the solid layer formed. The solidified areas can eventually break off the walls, mix with the coating fluid, and find their way onto the substrate, where they show up as flaws.
Trough type systems also typically use an overflow system which requires costly and complicated filtering of a catalyzed fluid. Overflow systems that use the top seals increase the pressure in the system and increase the likelihood of end seal leaks. A further problem caused by overflow and recirculation systems arises from the need to crosslink the polymers in the coating fluid. Since the effects of catalysts and other crosslinking agents are often cumulative over time, any eddies or areas of stagnation increase the average residence time in the fluid, thereby increasing the likelihood of premature chemical solidification. Furthermore, since clumps formed from catalyzed coating fluids often involve crosslinking or polymerization reactions, they are not likely to be redissolved by the coating fluid solvents. An additional problem which can occur is filter clogging since recirculating, clump-laden coating fluid typically passes through a filter, requiring more frequent cleaning or replacement of the filters. Eliminating overflow and recirculation reduces premature solidification by reducing fluid surface area and eliminates the need for recirculation and filtration of catalyzed fluids.
Coating apparatus which use fluid reservoirs and receiving rollers but which supply coating fluid to the reservoir without overflow and recirculation are known. A typical method for accomplishing this is to provide a fluid level sensor in the reservoir which feeds a signal back to the fluid supply source to control the rate of supply to the reservoir. U.S. Pat. No. 3,730,089 discloses a mechanical device which senses the size of the rotating vortex of ink in a printing press reservoir, thereby providing an indication of the ink level. The vortex sensor is mechanically coupled to an ink supply to provide additional ink as needed. However, this type of level sensor introduces additional surfaces on which dynamic wetting effects can occur, thereby increasing the rate of clump formation. A different ink level sensor is disclosed in U.S. Pat. No. 4,284,005 which measures the ink level in the reservoir by a capacitive device which senses the vertical location of the surface of the ink in the reservoir and sends an appropriate signal to the ink supply source. These capacitive devices are affected by nearby metal and are unreliable as it is hard to keep clean electrical connections.
Another approach to liquid depth measurement is to measure the hydrostatic pressure at the bottom of the liquid layer, such as with a bubble tube. Bubble tubes measure hydrostatic pressure in a liquid using a small tube which is connected to a pressurized supply of air or gas, called the test gas, placed in the liquid where the hydrostatic pressure is to be measured. The flow rate of the test gas is adjusted until a stream of bubbles form at the end of the tube. Measuring the gas pressure yields the hydrostatic liquid pressure, which provides an indication of the liquid level. Various types of bubble tubes and bubble tube improvements are disclosed in U.S. Pat. Nos. 2,668,438; 2,755,669; and 4,719,799. Bubble tubes and the gas supply systems needed to operate them are commercially available. A problem which sometimes arises when bubble tubes are used with solutions of solids in a volatile solvent is that premature solvent removal and solidification of the solution can occur on the surface of the tube. As bubbles exit the tube, the tube inside diameter becomes coated unless the test gas is solvent saturated. This can lead to clump formation and tube clogging.
One requirement for trough-supplied rotating rollers are reliable seals between the rotating surface of the receiving roller and the stationary reservoir. Many known end seal configurations are suitable for printing ink or other similar fluids. However, none can adequately withstand the high level of abrasiveness and exposure to solvents encountered in magnetic recording coating fluids and similar fluids. One method of sealing the ends of trough reservoirs, is disclosed in U.S. Pat. No. 4,945,832, includes sealing against the curved peripheral areas near each end of the receiving roller. The seal is made of closed cell silicone foam and provides sufficient flexibility to maintain contact with both the roller and the trough, while permitting the distance between the doctor blade and the roller to be adjusted. An additional seal contacts the end of the roller. However, when used with magnetic pigments, these seals are subject to severe wear and leakage. Additionally, wear products fall into the coating solution.
Coating apparatus which do not use reservoirs also are known. U.S. Pat. No. 4,332,840 discloses a variety of methods for dispensing coating fluid through a slot, called an extrusion bar, which extends across either the moving substrate or a receiving roller. The coating apparatus also includes additional rollers, doctor blades, or other metering devices. These devices for liquid delivery to rotating rollers involve some form of recirculation due to excess supply of the coating fluid.
During the coating process, coating must be stopped for periods of time ranging from minutes to hours. A typical cause for such stoppage might be web breaks, or malfunction of some part of the system. On such occasions, it is preferred to maintain the coating system in a standby mode, or idling state, to resume coating quickly when desired. It is also preferred to maintain the coating system in an idling state during the start of a coating run, since many adjustments and other tasks must be performed as part of setting up the coating process. However, if the coating apparatus is left idling for more than a few minutes, volatile solvents can evaporate from the reservoir, leading to excessive increases in viscosity and premature solidification of the coating fluid. This results in agglomerate formation and increased occurrence of coating defects when coating is resumed.
There is a need to provide a coating apparatus reservoir and a system for supplying coating fluid to the reservoir which reduces flaws in the coated product by reducing conditions leading to premature solidification due to solvent removal. There is a need for providing a system for sealing the ends of the reservoir to the roller which is less subject to wear and leakage than known devices. There also is a need to match the usage rate to the supply rate to the reservoir by eliminating the need for recirculation and to prevent defects from arising due to idle periods.